Heat-treatment of cobalt base alloys and products



Patented Nov. 1, 1949 HEAT-TREATMENT F COBALT BASE ALLOYS AND PRODUCTS Charles H. Savage, Morristown, N. J assignor to Crucible Steel Company of America, New York, N. Y., a corporation of New Jersey No Drawing. Application April 13, 1946, Serial No. 662,091

Claims. 1

This invention pertains to improvements in the heat-treating of cobalt base alloys, and to the resulting heat-treated products.

The alloys in question may contain up to about 40% chromium, up to about 1 carbon, and up to about molybdenum, with the balance substantially all cobalt, except for incidental impurities and small amounts of deoxidizing agents, such as manganese and silicon. Tungsten may be substituted for molybdenum in whole or in part. Up to about half the cobalt content may be replaced in whole or in part by nickel or iron, or both. In some cases, minor additions of columbium, tantalum, titanium, vanadium and zirconium, up to about 5% in aggregate, may be advantageous. Ordinarily, the alloys wil contain about 15 to chromium and about 0.1 to 0.5% carbon.

The excellent strength at elevated temperatures of cobalt base alloys, such as the above, has long been recognized. However, the brittleness evinced by these alloys at elevated temperatures has been a serious impediment to their extended use. Designing engineers naturally hesitate to specify a brittle alloy which may fail suddenly'with little or no warning. Heretofore, this brittleness has been accepted as an inherent and unavoidable quality of these alloys, since no treatment was previously known which would overcome it. However, I have evolved a heat-treatment which gives a critically higher elevated temperature toughness and ductility than was thought possible with these alloys, while, at the same time, introducing only a slight decrease in the elevated temperature strength thereof.

In the past, it was considered that only two kinds of heat-treatments could be advantageously employed for these alloys, via, a rapid cool from a high temperature substantially to room temperature; and, optionally, a subsequent precipitation-hardening treatment at a temperature around 1400 F. The rapid cool from a high temperature of approximately 2100 to 2400 F. was believed to place the alloy in the fully annealed or softest and toughest condition possible. The subsequent precipitation-hardening treatment at about 1400 F., where employed, was known to increase the hardness of the alloy appreciably, but generally at the expense of toughness.

I have discovered, however, in accordance with the present invention, that the toughness of these alloys may be remarkably increased over the results obtainable as aforesaid, either in the fullyannealed or in the precipitation-hardened condition, by subjecting the alloy to heat-treatment as follows: The alloy is first heated to a temperature between 2150 F. and its melting point, for a sufil- 2 held at this temperature for a sufficient period, usually for about an hour or so, to p rmit the alloy to transform, whereupon it is air cooled to room temperature. This transformation is ordicient period to insure maximum homogenization narily effected isothermally at a particular temperature; although it is sometimes advantageous to permit the alloy to transform by cooling it slowly through the aforesaid temperature of 1650 to 2100 F., rather than by holding it at a specific temperature. Such slow cooling, where employed, should be at such rate as to hold the alloy within the aforesaid temperature range for about an hour or so, before permitting it to air cool to room temperature.

The optimum temperatures for the two steps of the heat-treatment aforesaid may vary within the ranges given, depending on the analysis of the particular alloy to be heat-treated, and the desired properties. Ordinarily, a minimum temperature of about 2150 F. is required for the homogenization treatment, i. e., to dissolve most of the secondary constituents present in the alloy as rolled, as forged or as cast. Naturally, the alloy should not be heated above its melting point, since this would destroy the as-rolled or as-forged structure. The homogenization treatment is usually best carried out in the temperature range of about 2150 to 2350 F., and for a period of from a few minutes up to fifteen to thirty minutes or so, but seldom exceeding an hour. The higher the temperature, the shorter the heating time within these intervals.

Numerous tests have shown that the improvement in elevated temperature strength and ductility is obtained only if the second step of the heat-treatment is carried out Within the range of about 1650 to 2100 F. Transformation at temperatures under 1650" F. tends to have a deleterious eifect on the elevated temperature strength as well as on the ductility; while transformation at temperatures over 2100 F. is ordinarily impossible. A preferred temperature range for the isothermal transformation of the alloy is about 1900 to 2000 F. The length of the isothermal treatment is a function of composition of alloy and temperature of transformation, but in most instances one hour at temperature is sufficient, although holding within the transformation period for longer periods, 1. e., up to 16 hours or more, does no harm, but of course is uneconomical.

Generally, the product of the transformation is a lamellar constituent, although with some alloys, particularly those containing lower amounts of chromium, it may be more spheroidal in form. Usually, about one-third of the alloy transforms during the second or isothermal I step of the heat-treatment according to the invention. The coarseness of the lamellar structure increases with an increase in the tempera- PlllQ atwhichit is formed. On the basis of a 3 lon study of the microstructure. of these alloys, I believe the lamellar structure: is. the result of an eutectoid reaction. I am not prepared to treatments,.while tests 3. and 4 present results for heat-treatments according to the present invention.

Table I Analysis Per cent Carb 0.30 Man anese 0.68 N1 nlrp'l 2.63 Chromium" 27.40 Molyb n m 5.73 Balance cobalt:

1500 F. Stress Rupture Tests Room Test A I Temperature Heammtment Applied Rupture Elong., :32 Rockwell stress, time, per cent area Hardness p. s. 1. hrs. in 2 111. per cent 1 =2250 F.;,air cooled 40, 000 2; 871 9. 2 l5. 31 -32. 5 2 -2250 F.; air cooled plus 1400 F. for 16 hrs., 40,000 3. 77 13. 9 22. 5 38 '40 then air cooled. 3 2250 F; followedby an isothermal trans. 40,000 2.02 25.1 34.5 35 -36.-5*-

at.l800 F. for 1 hr., then air cooled. 4 2250 F; followed by an isothermal trans. 40,000 2.70 24.9 34.7 35. 536.5

at. 1900 F. for 1 hr., then air cooled.

state the composition of the" constituents of the: lamellar phase. Howevenit maybe readilydifferentiated metallographically from. the extremely fine precipitates: formed during precipitation-hardening at: temperatures from: about 1100 to 1500 F. Moreover, precipitation-hardening always'causesr a significant increase in the hardness of thezall'oy-overthat. obtainable by thev heat-treatment of; the present. invention- A, short time stresserupture. testihas been found. suitable to evaluate the strength and toughness at elevated temperatures ofthesealloys. Inthis test, the time for rupture is. determined. for a specimen heated toa specific temperature: and subjected to a. predetermined. stress. Either the rupture time for: a given stress or the stress re.- quired for a given rupture: time may be: used. as: the measure of the elevated temperature strength. I have. found it most convenient in comparing. the effects of various heat treatments to determine the. difference; in rupturetime ofv samples subjected: to: the same stress. The toughness is indicatedrby the reduction of area and the elongation. of; the failedspecimen. The? lower the reduction-of. area and' elongation, the more brittle is the: alloyi. The stress rupture tests were made at 1500 F.,. asthis is-a. common service temperature for cobalt base. alloys.

An example of. the: benefits of the" present invention is shown in Table I, wherein tests 1 and 2 represent the previously known types of: heat- Air cooling from 2250 F., per test 1, gives arupture time of 2.87 hours under a load: of ;.000 pounds per square inch at 1500 F., but the low elongation and reduction of area figures show that the material is very brittle in this condition. The precipitation-hardening treatment at 1400 F... per test 2; increases the elevated: temperature strength, as indicated by the rupture time, and. somewhat improves the elevated temperature ductility. It will be noted that the precipitation-hardening increases the room' temperature hardness considerably. The. effect of the two isothermal treatments in accordance with this invention, p'er tests 3 and 4, is startling. The strength at 1500 F. is slightly" lower than for tests 1 and 2, but the ductility is much greater. The elongation values are about 80 higher-for tests 3 and 4' according to the invention, than for the best of the prior treatments for tests 1 and 2,, while the reduction of area figures are over higher. I he isothermal transformation at 1900 F. is to be preferred to the isothermal transformation at 1800* F. for this alloy. as it increases the strength with practically nosacrifice of ductility. The increase in toughness and ductility thus obtainable ac"- cording to this invention will naturally greatly extend the possible uses of this alloy;

The following Table II gives the details of elevated temperature tests conducted on a similar alloy having a lower carbon content:

Table II analysis: Per cent Carbon M'nnzanese 0136 Nickel; 2.82 Chromi 27.35 Molybdenum 5.52 Balance cobalt.

1500 F. Stress Rupture Tests T t Room Tem es pcrature, N0; HeBtTreatmant Stress Rupture EIongL, Reduction Rockwell C Time, Per (ent of Area. Hardness.

' Hrs. ln2 in. Per Cent 1 2250 Fl. air cooled plus 1400 F. for 16 hrsL, then air cooled- 45, 000 1.95 4. 1 4. 3 39 40. 5 2250 F. followed by an isothermal trans.

. at 1700 F, for 1 hr., then air cooled. 45, 000 1.48 17. l 23. 7 33 34. 32250" F. followed'by'an'isothermal tra at 1900 F forl hr. then Mr c0oled': 45.000 l. 32 20'. 9' 2814 31. 5-32: 4:"... 2400" FL, air cooled plus 1400 F. for 16 40,000 3.87 2.4 2.9- 39 HO 5 hrs,, their air cooled 35, 000 6. 97 2.1 3.0 5 2400 F. followed by'an isothermal trans; 40. 000 2. 50' 13.4 18. 6 28 at 1700 F. for 1lir., then air cooled 35, 000 4. 40 13. 3 l9. 7

The beneficial eflect of the isothermal treatment is thus even more outstanding with this alloy. In the case of the first three tests made on specimens initially heated to 2250 F., the ductility is improved 270 to 410% under this inventionally large increase in elevated temperature strength of 260 to 370%, as evidenced by comparison of the "rupture time results at 38,000 pounds per square inch.

tion, tests 2 and 3, as judged on the basis of the Table I elongation values, and 450 to 560% on the basis Analysis; Per cent of the reduction of area figures, as compared with Z a the results obtained on the specimen which had flfi 3 33 been precipitation-hardened at 1400 F., per test fi g gg 1. In this particular case, the 1700 .F. isothermal 1500 F. Stress Rupture Tests T t Roomt'lem- GS para lllB, No. Heat Treatment stress Rupture Elong., Reduction Rockwell O S Time, Per Cent of Area, Hardness Hrs. in 2 in Per Cent 1 2250 F.,aircooled 45,000 1.37 7.1 8.9 24 2 2250 F in w 1 b i th 1 as'ooo 0 0 e y 31! S0 erma 45, 000 4. 93 19. 4 22. 9 w t11181231116561at 1900 F. for 1 hr., then an 381000 M83 20.0 23.5

treatment, test 2, gives the better strength While the 1900 F. isothermal treatment, test 3, gives the better ductility. However, even the 1700" F. treatment gives much higher ductility values than the precipitation-hardening treatment, test 1. The specimens initially heated to 2400 F, viz., tests 4 and 5, show a marked difference in ductility between those isothermally transformed, test 5, and those precipitation-hardened, test 4. The improvement in ductility resulting from the isothermal treatment is about 500%, as compared to the precipitation-hardening treatment. While the 2400 F. treatments decrease the ductility relative to the corresponding 2250 F. treatments, nevertheless the specimens heated to 2400 F. and then isothermally transformed, test 5, still have better ductility than the samples heated only to 2250 F. and then precipitation-hardened, test 1.

Additional results are shown in Table III below, for a still lower carbon alloy. Here the increase in ductility is over 600% at the cost of a small drop in strength for a heat-treatment under the present invention, test 2, as compared to those of the known precipitation-hardening treatment, test 1.

Cobalt base alloys of the above composition, and heat-treated in accordance with the present invention, have found successful elevated temperature applications for such uses as the buckets on turbine wheels of airplane superchargers, as well as on other forms of gas turbines. Numerous other applications suggest themselves where high strength, toughness and corrosion-resistance at elevated temperature are required.

I claim:

1. Process for heat-treating cobalt base alloys, which comprises: homogenizing at temperature above 2150 F. but below the melting point, and thereupon cooling to temperature within the range of about 1650 to 2100 F. and transforming within said range into a relatively soft, ductile and non-aged condition.

2. Process for heat-treating cobalt base alloys for increasing the elevated temperature toughness and ductility thereof, comprising the steps of: homogenizing at temperature above 2150 F.

, but below the melting point, and thereupon cooling to temperature within the range of about 1650 to 2100 F., holding and transforming the alloy within said range for at least one hour, and thereupon rapidly cooling substantially to room Table III Analysis: Per cent Garb0n 0.09 es 0. 59

Molybdenum 5: Balance: cobalt.

1500 F. Stress Rupture Tests T t RoomtTemes para ure, No. Heat Treatment stress Rupture Elong., Reduction Rockwell o s Time, Per Cent of Area, Hardness Hrs. inzin. Per Cent 1...... 2250 F., air cooled plus 1400 F. for 135 hrs., thcn air cooled 42, 000 2. 2. 7 3. 3 36-38 2 2250 F. followed by an isothermal trans.

at 1700 F. for 1 hr., then air cooled 42, 000 1. 12 18.9 24. 2 26-27 Table IV gives the elevated temperature properties for an alloy with about 10% nickel. The improved ductility resulting fromthe isothermal treatment, test 2, is 175 to 260% as evaluated on the basis of the elongation, and 175 to 365% on the basis of the reduction of area, as compared to the previously known treatment of test 1. Moreover, it is especially remarkable that this increased ductility is accompanied by an exceptemperature,- thereby to provide a relatively soft and ductile, non-aged microstructure.

8. Process for heat-treating cobalt base alloys for increasing the elevated temperature toughness and ductility thereof, which comprises: homogenizing at temperature within the range of about 2150 to 2350 F. for about two to thirty minutes, thereupon cooling to temperature within the range of about 1650 to 2100 r. and holding and transforming therein for atleast one hour, and thereupon cooling substantially to room. temperature, thereby to -provide a relatively soft and ductile, non-aged microstructure.

4. Process for heat-treating cobalt base alloys containing up to about'40'.% chromium, up to about 1% carbon, up to about 20% molybdenum, up to about 20% tungsten, nickel and iron up to about one-half the cobalt content, andthebalance substantially all cobalt, said process comprising the steps of: homogenzing the alloy at temperature above 2150 F. but below the melting point thereof, thereupon cooling to temperature within the range of about 1650. to 2100 F. and holding therein: for transformation into a relatively soft, ductile and non-aged state containing a microstructure of the group consisting of lamellar and spheroidal phases, and. thereupon cooling saidi alloy substantially to room-temperature.

5. Process for heat-treating cobalt base alloys containing up to about 40% chromium, up to about 1%- carbon, up .to about 20% molybdenum, up to about 20% tungsten, nickel and iron upto about one-half the cobalt content, and the balance substantially all cobalt, said process comprisin the steps. of; homogenizing at temperature above 215.0 F. but below the melting point and thereupon cooling, to within the temperature range of about 1650-to 2100 F., holding and transforming therein for at least one hour into a relatively soft, ductile and non-aged state containing amicrostructure of the group consisting of lamellar and spheroidal phases, and thereupon rapidly cooling, said alloy substantially to room temperature.

6, Process for heat-treating cobalt base alloys containing up to about 40% chromium, up to about 1% carbon, up to about, 20% molybdenum, up, to about 20% tungsten, nickel and. iron up to about one-half the cobalt content, and the balance substantially all cobalt, said process comprising the steps of: homogenizing at temperature within the range of about 2150 to 2350 F. for about two to thirty minutes, thereuponcooling to temperature within the range of about 1650 to 2100 F. and holding and transforming therein for at least one hour into a relatively soft, ductile and non-aged state containing a microstructure of the group consisting of lamellar and spheroidal phases, and thereupon cooling said alloy substantially to room temperature.

7. A heat-treated cobalt base alloy, characterized by high elevated temperature toughness and ductility, said alloy being in therelatively soft non-age-hardened condition resulting, from homogenizing at temperature above 2150 fol.- lowed by transforming within the temperature range of about 1650 to 2100 F.

8. A heat-treated cobalt base alloy containing chromium; from an effective amount-up toabout ditionresulting from homogenizing above 2150 F., followed by transforming in the temperature range of about 1650 to 2100 F.

9. A heat-treated cobalt base alloy, characterizedby hightoughness and ductility at elevated. temperatures, and containing chromium. in effective amounts up to: 40%, carbon up to about 1%, metal of the group tungsten and molybdenum up to about 20%, metal of the group nickel and iron up to about one-half the cobalt content, and the balance substantially all cobalt, said alloy being in the non-age-hardened, relatively soft condition resulting from homogenizing at temperature above 2150 F., followed by transforming in the temperature range of about 1650 to 2100 10. A heat-treated cobalt base alloy, characterized by high toughness and ductility at elevated temperatures, and containing; about 15' to- 25% chromium, about 0.1: to 05% carbon, metal of the group tungsten and molybdenum up to about 20 metal of the group nickel and iron up to about one-half the cobalt content, and the balance substantially all cobalt, said alloy being in the non-age-hardened, relatively soft condition resulting from homogenizing at temperature above 2150 R, followed by transforming in the temperature rangeof'about 1650 to 2100 F;

CHARLES H. SAVAGE.

REFERENCES CITED The following references. are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions, American Institute of Mining and Metallurgical Engineers, Institute of Metals Division, vol. 83, pages 314, and 320 to 325. Published in 1929 by A. I. M. E. New York.

Transactions, American Society for Metals, vol.

23, pagQS.2 73 t0 2'76. Published in 1935 by Ameri;

can Society for Metals, Cleveland, Ohio. 

